What happens if you put 90w in the transmission but haven't replaced the mainshaft bearing with the sealed set? Replacing the bearing with a sealed unit is something I'd like to do, but I'm hoping to ride for the season before tearing into the transmission, but I don't want to ride it if the oil is going to leak everywhere... What's the word?

If you are just topping it off with gear oil, it will probably be okay when it gets mixed with the 00 grease that's inside.If you have already removed all the grease out of it, then it probably will leak with just the gear oil inside.

Just do the sealed bearing upgrade. It's actually quite easy. Took me about 3 hours, including cleaning up the nasty grease all over everything. But I'm running gear oil only in there right now, and nary a leak since reinstallation. I check the levels every now and again, just to make sure it's not leaking out the main shaft on the primary side by the chain (just about the only place it could get out that I can't see easily). Never had to do more than the most minimal top off.

I was just hoping to get a season out of it, hoping the leak would be really, really slow and I could just top it off. You know, a drop or two. Sounds like I need to order the kit. And I just did an order, so now I can't get the free shipping! Dang it! Any other mods that are urgent, while I'm putting in this order?

Well, if it runs and goes forward, then no mods are ACTUALLY necessary.

That said, if you have left side shift, the upgrade kit with careful installation does give a more positive shift action to the Goldberg shift mechanism on these old bikes. I did one, and it has been a very good improvement.

I can't tell you how slow or fast the gear box would leak if you continued to just top off in the grease they use stock in there. You might be able to go all season. But, for me, having owned a Norton prior to my enfield, I was interested in trying to reduce oil puddles. I have successfully done so, via several upgrades. Including the one we are talking about.

What happens if you put 90w in the transmission but haven't replaced the mainshaft bearing with the sealed set? Replacing the bearing with a sealed unit is something I'd like to do, but I'm hoping to ride for the season before tearing into the transmission, but I don't want to ride it if the oil is going to leak everywhere... What's the word?

Like Ace said, if you're just adding some oil to the box you'll be fine. If you're cleaning the box of grease and re-filling with oil, you'll leak.

Given you said you don't want to tear into the transmission, I'm guessing you want the former and not the latter.